MFS11 r342229: bootpd: validate hardware type

Due to insufficient validation of network-provided data it may have been
possible for a malicious actor to craft a bootp packet which could cause
a stack buffer overflow.

admbugs: 850
Reported by: Reno Robert
Reviewed by: markj
Approved by: so
Security: FreeBSD-SA-18:15.bootpd
Sponsored by: The FreeBSD Foundation